Class I molecules of the major histocompatibility complex (MHC) consist of a highly polymorphic heavy chain complexed to b2-microglobulin (beta2m). Class I molecules are constitutively expressed by numerous cell types in the body. Expression by non-expressing cell types occurs rapidly in the course of a immune response following exposure to g-IFN and other cytokines. The sole function of class I molecules is to bind antigens and present them to T cell bearing CD8 molecules (TCD8+). TCD8+ play a critical role in eradicating intracellular pathogens and tumors. They can also contribute to immunopathology, being involved in organ rejection and autoimmune diseases. There has been rapid progress in understanding the physical nature of the antigen-class I complex, and in how antigens are generated and become associated with class I molecules in cells. Peptides of 8 to 15 residues produced from a cytosolic pool of proteins by cytosolic proteases are translocated into the endoplasmic reticulum (ER) by a MHC encoded transporter complex known as TAP. Once in the ER, peptides (possibly after further trimming by peptidases) bind to class I molecules associated with TAP and are transported to the cell surface. In the past year we have continued our studies the assembly and trafficking of MHC class I molecules and have made progress on a number of fronts: 1) We developed fluorescent derivatives of an antigenic peptides and used these probes to make several novel findings about the intracellular trafficking of class I molecules and antigenic peptides, 2) We have studied how antigenic peptides are generated from proteins targeted to the ER by inserting a N-linked glycosylation site in a secreted protein. This revealed a novel intracellular pathway by which proteins are returned from the ER to the cytosol.